The present invention, in some embodiments thereof, relates to medical application and, more particularly, but not exclusively, to a method and system for estimating exercise capacity.
Congestive Heart Failure (CHF) is a chronic inability of the heart to maintain an adequate output of blood from one or both ventricles of the heart to meet the metabolic demands of the tissues. In patients diagnosed with CHF, sudden cardiac death occurs at 6 to 9 times the rate of the general population.
In CHF subjects having a markedly weakened left ventricle and not markedly weakened right ventricle, blood continues to be pumped into the lungs but is not pumped adequately out of the lungs. As the volume of blood in the lungs increases, the pulmonary vessels enlarge, pulmonary venous congestion develops, and, once the pulmonary capillary pressure rises above a critical point, fluid begins to filter out of the capillaries into the interstitial spaces and alveoli, resulting in pulmonary edema. Pulmonary edema may lead to pleural and abdominal effusion.
In CHF subjects having abnormality in the right heart or the pulmonary arteries, the ability of the heart to move blood forward is limited and congestion occurs behind the right heart. Such congestion causes pleural effusion and/or buildup of fluid in the abdomen.
Numerous techniques for predicting and determining the condition of CHF subjects are known. One such technique is known as exercise capacity test. In exercise capacity test, the respiratory and/or circulatory response of the subject to exercise is analyzed to stratify risk and assess therapy. Analysis of expired gas during exercise is commonly known as cardiopulmonary exercise testing (CPX) or metabolic exercise.
CPX typically includes the calculation or estimation of several measures. Oxygen uptake, also known as VO2, is the rate of oxygen consumption by a patient during an exercise test. Peak VO2 is considered as a measure of a patient's aerobic exercise capacity, and has traditionally been considered a surrogate for maximal cardiac output. Carbon dioxide production rate, also known as VCO2, is the rate of carbon dioxide production by a patient during exercise. VCO2 relative to VO2 is influenced by which substrate is metabolized (fat vs. carbohydrate) and whether anaerobic processes and lactic acid production occur. Known in the art are methods of calculating VO2 and VCO2 by numerical integration of the product of expiratory airflow with O2 and CO2 concentrations.
Attempts have been made to developed techniques for estimating cardiac output during exercise. Generally, direct measurement of cardiac output by thermodilution is considered the “gold standard.” Yet, this technique is cumbersome, invasive, and carries an added expense and degree of risk. Several non invasive techniques are described in Leslien et al., “Non-invasive measurement of cardiac output in patients with chronic heart failure,”, Blood Press Monit 2004, 9:277-280; Engoren et al., “Comparison of cardiac output determined by bioimpedance, thermodilution, and the Fick method,”, Am J Crit. Care 2005, 14:40-45; and Newman et al., “The non-invasive assessment of stroke volume and cardiac output by impedance cardiography: a review,” Aviat Space Environ Med, 1999, 70:780-789.